A plated steel sheet with plating of an aluminum-zinc alloy (aluminum-zinc plated steel sheet) has a high corrosion resistance compared with a hot-dip galvanized steel sheet. A steel sheet plated with an aluminum-zinc alloy having an aluminum percentage of about 55% by mass, i.e., a high aluminum-content zinc plated steel sheet, has particularly high corrosion resistance, and also has excellent heat resistance and heat reflectivity. Thus, in recent years, aluminum-zinc plated steel sheets have become widely used in building material products such as roofing materials and wall materials, construction materials such as guardrails, sound insulating walls, snow fences, and gutters, materials for cars, home appliances, and industrial equipment, and moreover for use in bases for painted steel sheets.
As a result of the aluminum-zinc plated steel sheet being painted, it has much higher corrosion resistance.
However, the aluminum-zinc plated steel sheets are sometimes stored temporarily before the aluminum-zinc plated steel sheets are painted, resulting in the occurrence of black rust or white rust. If the aluminum-zinc plated steel sheet is stored in a high temperature and high humidity atmosphere, blackening may also occur. In particular, if water droplets adhere to the surface of the aluminum-zinc plated steel sheet due to condensation, blackening is likely to occur selectively in the areas where the water droplets adhere. As a result, not only does the external appearance of the aluminum-zinc plated steel sheet deteriorate, but also the surface composition is not uniform, as a result of which corrosion resistance is reduced and if painting is performed, the adherence of the aluminum-zinc plated steel sheet to a coating film formed by painting is reduced.
Therefore, conventionally, a surface treatment for improving corrosion resistance and blackening resistance is performed on the aluminum-zinc plated steel sheet. In the past, a chromate treatment, a treatment for forming a resin covering film including chromium, or the like has been performed. However, recently there is a demand not to use chromium from the point of view of environmental protection and the like, and thus an attempt has been made to use a surface treatment agent that does not contain chromium.
For example, JP 2003-201578 A (hereinafter, referred to as Document 1) discloses that a covering film is formed from a surface treatment agent containing an urethane resin, N-methylpyrrolidone, a zirconium metallic compound, and a silane coupling agent. JP S57-39314 B (hereinafter, referred to as Document 2) discloses that a protective coverage is formed from an acidic solution having a pH of 2 to 4 that contains at least one of Ti salts and Zr salts, and H2O2 and at least one of phosphoric acid, condensed phosphoric acids and derivatives of phosphoric acid. JP 3992173 B2 (hereinafter, referred to as Document 3) discloses that a treatment is performed on a metal surface using a non-chromate-type composition for metal surface treatment containing a metal acetylacetonate and at least one of a watersoluble inorganic titanium compound and a water-soluble inorganic zirconium compound in a specific ratio.
However, with the method described in Document 1, if alkaline cleaning is performed on the aluminum-zinc plated steel sheet before painting, the covering film is sometimes partially peeled off, resulting in unevenness in the appearance thereof after painting. The covering films formed by the methods described in Documents 2 and 3 contain a large amount of soluble salts such as phosphorus compounds and fluorine compounds. Therefore, the covering film is likely to elute soluble salts in a high temperature and high humidity atmosphere. Furthermore, these soluble salts are likely to be eluted therefrom in alkaline cleaning. Thus, the corrosion resistance and blackening resistance of the aluminum-zinc plated steel sheet are reduced.